This invention relates to fuels. More specifically, this invention relates to quadricyclane and a highly efficient process for producing quadricyclane.
Quadricyclane, or tetracyclo[2.2.1.02,6.03,5]heptane, is known in the prior art as a chemical for energy storage, in particular, solar energy storage. Quadricyclane has also been recognized as a potential fuel for internal combustion engines, as described in U.S. Pat. No. 5,076,813; and as a rocket propellant, as described in U.S. Pat. No. 5,616,882. In such applications, quadricyclane may be used alone or as a component along with other hydrocarbon fuels for the purpose of modifying the fuel characteristics, e.g., increasing the heat of combustion, or fuel density.
Quadricyclane has been identified as a possible high-energy replacement for, or additive to, current hydrocarbon-based rocket propellants. Quadricyclane""s highly strained structure gives it a specific impulse and density that is greater than current hydrocarbon-based rocket propellants. It is hoped that replacing current rocket propellants with quadricyclane will enable launching payloads having greater mass on otherwise equivalent launch vehicles.
Currently, the accepted synthesis of quadricyclane is via a photochemical transformation from norbornadiene, bicyclo[2.2.1]hepta-2,5-diene.This transformation may be carried out by direct ultraviolet radiation of purified or unpurified norbornadiene utilizing mercury arc lamps. The transformation may be made more efficient by using a sensitizer molecule that absorbs the ultraviolet light and transfers its energy to the norbornadiene. However, it has been determined that the use of some prior art sensitizers simultaneously leads to a significant decrease in the rate of quadricyclane production with increased conversion. Furthermore, the addition of more sensitizer in an effort to gain better efficiencies in the production rate is not preferable, particularly in continuous flow reactors, because of the additional steps required.
Sensitizers for the transformation of norbornadiene to quadricyclane have been studied for almost 40 years. Benzophenone, 4-dimethylaminobenzophenone, 4,4xe2x80x2-bis(dimethylamino)-benzophenone (Michler""s Ketone), as well as various copper salts have been reported to be relatively efficient sensitizers for the reaction. As mentioned previously, ultraviolet light sources to drive the photochemical transformation have included sunlight, simulated sunlight (for solar energy storage applications), and both medium and high-pressure mercury arc lamps.
Notwithstanding the prior efforts and developments in the production of quadricyclane, what is needed is a less expensive and more efficient process for carrying out the solution phase photochemical transformation of converting norbornadiene to quadricyclane.
It has therefore been an object of the present invention to provide a less expensive and more efficient process for carrying out solution-phase photochemical transformations.
It has also been an object of the present invention to provide a less expensive process for converting norbornadiene into quadricyclane.
It has been a further object of the present invention to provide a more efficient process for converting norbornadiene to quadricyclane.
Another object of the present invention is to provide relatively inexpensive quadricyclane manufactured by the process described herein in order to provide a relatively low cost yet high-energy fuel.
The objects of the present invention are achieved by irradiating a solution of purified or unpurified norbornadiene and a sensitizer having a solubility in norbornadiene greater than that of Michler""s Ketone with light emissions from a light source having wavelengths in the range of about 250 nm to about 400 nm and, preferably, in the range of 340 nm to 390 nm. Mercury arc lamps, including both medium and high-pressure arc lamps, in which the lamp is doped with small amounts of other elements enhance the emission of the mercury arc in the near-UV (UVA), region of the spectrum. These doping additives have the ultimate effect of increasing the efficiency of light output at wavelengths useful for photochemical transformations when used both in conjunction with and without the aid of a sensitizer.
In a preferred embodiment, metal halide-doped mercury arc lamps and, specifically, iron halide-doped mercury arc lamps, that have enhanced outputs in the range of about 340 nm to about 390 nm are used to enhance the sensitized conversion of the norbornadiene to quadricyclane. The enhanced emission wavelength of an iron halide-doped mercury arc lamp in the range of about 340 nm to about 390 nm overlaps with the absorption of diaminobenzophenone sensitizers, e.g., Michler""s Ketone. It has been found that the use of an iron halide-doped mercury arc lamp provides an increase of about 20% in the conversion rate of norbornadiene to quadricyclane when used with Ethyl Michler""s Ketone as the sensitizer, as further discussed below. The photochemical transformation is preferably carried out at a temperature in the range of about xe2x88x9240xc2x0 C. to about 60xc2x0 C.
Also in a preferred embodiment, the sensitizer in the conversion of norbornadiene to quadricyclane is Ethyl Michler""s Ketone (4,4xe2x80x2-bis(diethylamino)benzophenone). Use of Ethyl Michler""s Ketone as the sensitizer has led to a decrease in the induction period at the beginning of the conversion reaction, and an increase in the photon or quantum efficiency of conversion of norbornadiene to quadricyclane.
The result of replacing Michler""s Ketone with Ethyl Michler""s Ketone reduces or eliminates the requirement of using purified norbornadiene to obtain the highest conversion rates to quadricyclane. The use of Ethyl Michler""s Ketone improves the conversion rate of norbornadiene to quadricyclane by about 40% when compared with the use of Michler""s Ketone as a sensitizer. Also, the addition of triethylamine to the reactor reduces the amount of side products formed during the transformation, thereby reducing downtime.
Because there is no significant increase in cost to drive the photochemical transformation by using an iron halide-doped mercury arc lamp and/or Ethyl Michler""s Ketone and/or triethylamine, it is significantly less expensive to produce quadricyclane by the present inventive process. Thus, with the present inventive process it is more efficient and less expensive to produce quadricyclane, ultimately potentially allowing lower cost access to outer space.